Superplasticity of bulk metallic glasses (BMGs): A review

Under specific deformation conditions, thermoforming in the supercooled liquid region (SLR) might lead to Newtonian flow characterized by high strain-rate-sensitivity-index (m) of ∼1 and achieving superplastic ductilities. Both incipient deformation-induced crystallization and rapid increase in the stress-assisted free volume lead to decreased m-values and transition to non-Newtonian flow. The maximum elongations can be achieved near the transition strain rate from Newtonian to non-Newtonian flow. The effects of heating rate to the thermoforming temperature, total processing time, and the extent of the SLR are significant for structural stability against the in-situ crystallization during elevated-temperature hot deformation. Increasing the deformation temperature normally accentuates the superplastic behavior, but high temperatures might promote crystallization and loss of superplastic ductility . Future prospects for research have been recognized as optimization of the volume fraction of the crystalline (reinforcement) phase in BMG composites, superplasticity of high-entropy BMGs, thermoplastic micro-formability, ultrasonic-assisted forming, improved thermoplastic formability by alloying method, and superplasticity of BMG parts fabricated by additive manufacturing processes.